Out of curiosity, I went to the Bob Cropp testimony which was all about defending big co-ops. Dr.Cropp begins his defense on page 191. He compares the size of co-ops with big retailers.
Then he talks about the over-order premium negotiated by co-ops on page 194, which he says averaged $2.45. I don't think that is right. See: http://www.ams.usda.gov/AMSv1.0/getfile?dDocName=STELPRDC5081678
Here in the Northeast, with high class I use the premium is $1.50. In Wisconsin, with little class I use the premium is $3.52. Where the big co-ops have power there is a small premium. Where there is competition, the premium grows.